1. Technical Field
The present invention relates to an art for suppressing feedback by use of an adaptive filter.
2. Background Art
Occurrence of feedback causes problems in many cases in the field of an acoustic feedback system that amplifies a signal of sound collected within single acoustic space by means of a microphone and that emits the thus-amplified signal from a speaker. An anti-feedback device utilizing an adaptive filter is available as means for suppressing such feedback. Such an anti-feedback device generates from a signal input to the speaker a simulated signal that simulates a circulatory sound component, which originates from a speaker and enters a microphone, by means of an adaptive filter. The simulated signal is cancelled out by the signal output from the microphone. However, when a change arises in the state of a transmission system of circulatory sound, the adaptive filter consumes much time before outputting a simulated signal that accurately simulates circulatory sound achieved after occurrence of a change in the state of the transmission system. For this reason, the anti-feedback device utilizing an adaptive filter encounters a problem of being unable to sufficiently suppress feedback in a situation where an abrupt change arises in the state of the circulatory sound transmission system. The anti-feedback device utilizing an adaptive filter also encounters a problem of so-called coloration arising when the adaptive filter has insufficient accuracy in estimation of circulatory sound or when a change arises in positional relationship between a speaker and a microphone.
Patent Document 1 and Non-Patent Document 1 disclose arts using an adaptive filter and a notch filter in combination as an art for enhancing suppression of feedback. An anti-feedback device described in Patent Document 1 suppresses circulatory sound components by means of an adaptive filter. When feedback occurs, a notch filter performs processing for attenuating a component of frequency at which feedback arises by means of a signal acquired by way of a microphone. An anti-feedback device described in Non-Patent Document 1 suppresses a circulatory component by means of an adaptive filter of PEM-AFROW type. A notch filter performs processing for estimating a frequency at which a transmission system that connects a speaker to a microphone exhibits a peak and for attenuating the thus-estimated frequency component by means of a signal acquired by way of the microphone.
[Patent Document 1] JP-A-2006-217542
[Non-Patent Document 1] G. Rombouts, T. Watershoot, M. Moonen, “Proactive notch filtering for acoustic feedback cancellation,” Proc 2nd Annual IEEE Benelux/DSP Valley Signal Process. Symp. April 2006, pp. 169-172
In the art described in Non-Patent Document 1, appropriate suppression of feedback requires an adaptive filter whose filtering coefficient accurately reflects an amplitude characteristic of a closed loop. To this end, updating a filtering coefficient requires a large amount of arithmetic calculation, which raises a problem of difficulty in enhancing the speed of anti-feedback processing.